1. Field of Invention
The present invention relates to a control circuit, and in particular to a control circuit of a sensorless motor and a control method thereof.
2. Related Art
In general, a sensorless motor operates according to an induced electromotive force of an induction coil. FIG. 1 is a schematic illustration showing a conventional control circuit 1 of a sensorless motor. Referring to FIG. 1, the control circuit 1 includes a first switch element 11, a second switch element 12, a third switch element 13, a fourth switch element 14, a coil 15, a positive power input terminal VDD and a ground Vground. The positive power input terminal VDD supplies the power required by the control circuit 1.
The coil 15 is disposed around several silicon steel sheets (not shown) of the motor, and the currents flow through the coil 15 via different directions so that the silicon steel sheets generate different magnetic forces to repel a magnetic pole of a rotor of the motor so as to push the rotor. The control circuit 1 operates as follows. First, the first switch element 11 and the fourth switch element 14 are turned on to make the current flow through the coil 15 via the first switch element 11 and the fourth switch element 14. Meanwhile, the induced electromotive force is detected from two ends A and B of the coil 15 so that the magnetism of the magnetic pole corresponding to the coil 15 can be determined. If the magnetism of the silicon steel sheets has to be changed, the control circuit 1 turns on the second switch element 12 and the third switch element 13 so that the current flows through the coil 15 in a different direction and the magnetism of the silicon steel sheet is changed to continue to push the rotor of the motor.
Because the magnetism of the silicon steel sheets has to be continuously changed, the control circuit 1 must detect the induced electromotive force of the coil 15 at the two ends A and B of the coil 15 so that the motor can operate according to the induced electromotive force. In order to detect the induced electromotive force of the induction coil 15, the coil 15 has to be powered off for a period of time, which decreases the efficiency of the motor. In order to improve the efficiency of the motor, the control circuit 1 has to predict the rotating speed of the motor at the next time when motor starts to operate again to adjust the time from powering off to starting switching the current. However, the error of the control circuit 1 inevitably occurs, and the current cannot be immediately cut off so that the power is consumed. In addition, the noise of the motor is too high when the coil is powered on at the next time.